Filament transformer



Nov. 16, 1943. .Y A. w. vANcE Erm.

FILAMENT vT1' 1|.NsFommR Filed .nm so. 1942 Zmnentor Gttorneg Patented Nov. 1s, 1943 A,

FILAMENT TBANSFORMEB.

Arthur W. Vance, Moorestown, and Jeremiah M. Morgan, Haddoniield, N. J., assignors to Radio Corporation oi' America, a corporation of Del- Application June 30,

6 Claims.

This invention relates to an improved transformer unit for use in supplying radio frequency heating current to the filament of an electron microscope or other apparatus wherein the filament or cathode is normally operated at a high potential (e. g. 50-100 kilovolts) with respect to ground.

The use of radiol frequency instead of direct l current in high voltage supplies and current regulating systems for electron microscopes and othertance surge resistor on the secondary side of the transformer, (b) the high potential difference between the primary and secondary windings of the transformer, and (c) between the filament and the frame (i. e. ground) of the cabinets or compartments wherein the filament current is generated and utilized.

Accordingly, the principal object of the present invention is to provide an improved device for supplying radio frequency heating current to the filament of an electron microscope or like apparatus wherein the filament or cathode is normally operated at a high negative potential with respect to ground. Y

' Another and important object of the invention is to provide an improved device of the general character described and one incorporating, in a single completely shielded demountable unit, a filament transformer together with the necessary resistors, leads, terminal connectors and other circuit elements necessary to the efficient transfer of the transformed current to a point remote from its point of origin;

Other objects and advantages together with certain preferred details of construction will be apparent and the invention itself will be best understood by4 reference to the following specification and to the accompanying drawing, wherein:

Figure 1 is a circuit diagram of a radio frequency transformer to which reference will be made in describing the present invention, and

Figure 2 is a longitudinal sectional view of a demountabler--f transformer unit embodying the invention, and

Figure 3 is an enlarged fragmentary sectional view of the transformer primary winding. In the drawing like reference characters desig- 1942, serial No. 449,112

nate the same or corresponding parts in both figures.

In Figure 1, P designates generally a cabinetv which may contain a complete power supply sys- 5 tem, but which is here indicated as containing l'simply those elements necessary for supplying radio frequency heating current to the filamentarir cathodeF of an electron microscope M or like apparatus wherein the filament is normally operated at a high negative potential with respect to ground. A filament transformer comprising windings I5 and 29 within the cabinet P has a tuned primary circuit which may comprise the inductor l5 of a tank circuit Il, l5 which will be understood to be connected to an oscillator not shown, but which may be similar, to the one described in copending application Serial No. 365,750 to Arthur W. Vance, filed November 15, 1940. As described in the Vance application this filament transformer should have a low interwinding capacity of low power factor. Also, the primary and secondary windings I5 and 29, respectively, must be spaced suiliciently to withstand the peak voltage output, and, as a result, a large 'amount of energy must be stored in the tuned primary I5 in order that the secondary 29 may absorb the power necessary to excite the filament F. The size of the secondary 29 is selected to match the impedance of the filament loadand 9 to provide the desired voltage. The radio frequency current supplied by the oscillatory circuit is shown as applied to the filament F through a twisted pair 33, though, as described in the above-identified Vance application, a coaxial cable may be employed for this purpose.

Referring still to Fig. 1, the high voltage for l the microscope is shown as derived from a suitable source of direct current which is indicated symbolically at V, and applied to the filament F through a surge resistor 35, and a center tap resistor 3l. A branch conductor 49 which extends from a point intermediate the resistors 35 and 31 to a sheath 45 about the filament leads 33 maintains the said Vsheath at the same highly negative F. It will be observed that a filter capacitor C on the input side of the surge resistor 35 serves to by-pass disturbing ripple voltages which may there be present. The capacitor C, however, is entirely ineffective in suppressing any undesired voltages which may be picked up either by the said resistor or by the leads or other apparatus on the output side of said resistor. Accordingly, as described below in connection with Figure 2, the present invention provides a shielding system which embraces all of the said parts.

The shielded transformer unit of the invention as shown in Fig. 2 comprises a hollow cylindrical casing l made of hard rubber or porcelain or the like and is provided on its outer surface with a potential, with respect to ground, as the filamenty dry film like coating 3 constituted of a semi-conductor such as colloidal carbon (Aquadag) or, less desirably, with a metal screen, not shown. A metal collar 5 in contact with the shield 3.on the outer surface of the csaing I, intermediate its ends, supports the said structure in an aperture 1 which is provided for the purpose in the grounded frame 3 of the power supply cabinet. The collar 5 may further comprise a support for the tank capacitor II of Fig. 1. An annular recess I3 in the outer surface of the insulating structure I comprises a seat for the primary winding I5 ofthe tuned circuit II, I5 of Fig. 1. As shown in Fig. 3 the primary winding I5 may comprise a layer wound Litz wire coil with kraft paper insulation I1 between the layers to provide a low inter-winding capacitance of low power factor.

A metal bushing I9 which extends into the bore of the insulating cylinder I supports a jack 2| which is adapted to receive a plug or other connector (not shown) for the high voltage source V (Fig. 1). The opposite end of the jack 2| terminates in an integralA metal cylinder 23 over which end of an insulating sleeve 25 is fitted and secured by screws 21. This insulating sleeve 25 may be constituted of a hard rubber or of a synthetic resinous material, (e .g. Bakelite) and preferably extends from the jack 2I to or beyond the opposite end of the bore of the casing I. The outside diameter of the said sleeve or hollow conduit 25 is sufficiently small to serve as a support for the secondary winding 29 -of the transformer I1, 23. The secondary 23 preferably comprises a flat metal ribbon which is wound on the sleeve or -tubing 25 prior to its insertion within the bore of the casing, in a position to bring it in register with the primary winding I1. The opposite ends of the ribbon like secondary winding 23 are fixed to grommets 3l and 3Ia, respectively, which serve to connect them to the transmission line 33 which extends along the interior of the conduit 25 to the exterior of the casing I.

As brought out in connection with Fig. 1, the high voltage from the source V (Fig. v1) is applied to the filament leads 33 through a surge resistor 35 and a center tap resistor 31. In accordance with the invention, these resistors are mounted within the inner conduit 25. Thus, as shown in Fig. 2, the surge resistor 35 is mounted within the central conduit 25 adjacent to the end thereof and is connected as by solder to the inner end of the jack 2I. 'I'he center tap resistor 31 is also mounted within the said conduit-25, preferably in register with the secondary winding 29 and connected to the surge resistor 35 as by short wires 33, as shown.

To prevent arc-over between the filament leads 33 and the conductive sheath 4I which surrounds the said leads in the space between the end of the transformer unit I and the microscope chamber 43 it is preferable to maintain the said sheath at the same high potential as the filament leads. This is achieved in accordance with the invention by the provision of a conductive sheath 45 mounted in the space between the inner surface of the insulating casing I and the'outer surface of the insulating conduit 25. In the illustrated y embodiment of the invention this conductive sheath extends from a point adjacent to the terminals 3| of the secondary winding 23,'outwardly to a metal nut 41 which may serve to connect it to the external section 4I surrounding the filament leads. The connection from the inner section 45 of the sheath (4I, 45) .to thehigh voltage source is completed through a wire 49 which is shown connected directly -to the surge resistor 35 to avoid the effects of a possible short circuit upon the center tap resistors.

As indicated at 5I in Fig. 2 it is desirable to provide the otherwise unfilled voids within the casing I and'conduit 25 with a filling of wax or other insulating, moisture-proof substance.

From the foregoing, it will be apparent that l the present invention provides an improved and compact completely shielded readily demountable unit containing a filament transformer together with the necessary resistors, leads, terminal con-4 face of said casing, a transformer primary winding insulatingly supported on said shielding means,V an insulating conduit mounted within and in spaced relation with respect to the inner surface of said casing, a transformer secondary winding in the space between the outer surface of said conduit and the innersurface of said casing, a transmission line conlnected to said secondary winding and extendingI through said conduit to the exterior of said insulating casing, and means within said conduit for connecting said transmission line to av high voltage source.

2. The invention as set forth in claim 1 and wherein said transmission line is connected to said secondary winding through a pair of center tap resistors mounted within said conduit.`

`3. The invention as set forth in claim 1 and wherein said conduit contains a surge resistorin series with said high voltage connecting means.

4. The invention as set forth in claim l and wherein said insulating casing comprises a bushing and is provided on its outer surface with means for attaching said bushing to a supporting panel.

5. The invention as set forth in claim 1 and wherein said high voltage connecting means ter- `minates in a jack which extends into the interior spect to said primary winding in the space between the outer surface of said conduit and the inner surface of said casing, a transmission line connected to said secondary winding and extending through said conduit to the exterior of said casing, a conductive sheath about said transmission line in the space between the outer surface of said conduit andthe inner surface of said casing, and means Within said conduit for connecting siad transmission line and said conductive sheath to a high voltage source. y

ARTHUR W. VANCE. JEREMIAH MORGAN. 

